This invention relates to moldings based on pyrogenically produced mixed oxide, to a process for the production thereof, and to the use thereof as a catalyst support or as a catalyst.
Pyrogenically produced oxides are distinguished by extreme fineness and a correspondingly elevated specific surface area, very high purity, spherical particle shape, and the absence of pores. Due to these properties, there is increasing interest in pyrogenically produced oxides as supports for catalysts (see, for example, D. Koth, H. Ferch, Chem. Ing. Techn. 52, 628 (1980)). In some cases, pyrogenic oxides are also used as catalysts.
Because pyrogenically produced oxides are particularly finely divided, shaping them into catalyst supports or catalysts occasions some difficulties.
DE-A 31 32 674 discloses a process for the production of compression moldings from pyrogenically produced oxides in which silica sol is used as the binder. This document is incorporated by reference herein in its entirety.
DE-A 34 06 185 discloses a process for the production of compression moldings in which glaze frit powders are used as a binder and glycerol as a lubricant. This document is incorporated by reference herein in its entirety.
DE-A 39 12 504 discloses a process for the production of compression moldings in which aluminum stearate, magnesium stearate and/or graphite are used as lubricants, and urea together with methylcellulose are used as pore formers. This document is incorporated by reference herein in its entirety. These compression moldings are offered for sale as Aerosil(copyright) tablets no. 350, available from Degussa. They contain approx. 0.4 wt. % Mg.
Moldings based on pyrogenic oxides are known. EP-B 0 393 356 describes high purity moldings based on pyrogenically produced silicon dioxide. (This document is incorporated by reference herein in its entirety.) These moldings comprise  greater than 99 wt. % of pyrogenically produced SiO2 and have a low binder material content. The Examples in this patent document describe the use of Mg and Al stearates.
DE-A 196 19 961 describes moldings based on pyrogenically produced silicon dioxide. The purity of these moldings has been increased to  greater than 99.8 wt. % of SiO2. This document is incorporated by reference herein in its entirety.
Documents EP-B 0 452 619, EP-B 0 394 677 and EP-B 0 327 723 describe moldings based on pyrogenically produced titanium dioxide. (These documents are incorporated by reference herein in their entirety.) These moldings are differentiated inter alia by their content of anatase and rutile, by purity, by dimensions, etc. Titanium dioxide contents are stated as  greater than 99 wt. %,  greater than 99.5 wt. % and  greater than 94 wt. %. In the latter case, the content of anatase is at least 50 wt. %, with the remaining 0-6 wt. % comprising non-pyrogenically produced SiO2 and/or non-pyrogenically produced Al2O3.
EP-B 0 327 721 describes moldings based on pyrogenically produced Al2O3. This is document is incorporated by reference herein in its entirety. Purity is stated as a content of 94-99 wt. % Al2O3. The remaining 1-6 wt. % are SiO2. The SiO2 fraction originates from kaolin, which is used during shaping of the aluminum oxide powder.
Moldings based on pyrogenically produced ZrO2 also are known. Such materials are described in the document EP-B 0 327 814, which is incorporated by reference herein in its entirety. The purity of these materials is stated as  greater than 92 wt. % of zirconium dioxide. In this case too, the remainder contains fractions of non pyrogenically produced SiO2 and Al2O3. Small fractions of HfO2, which is a natural impurity in ZrO2, are also present.
Known moldings based on pyrogenically produced oxides may be used as catalysts or catalyst supports. The elevated purity and good mechanical and chemical properties play an important part in many applications.
It is furthermore known to modify the composition of the moldings for particular applications by purposefully adding certain foreign elements. Examples of this are described in the document EP-A 0 723 810, which is incorporated by reference herein in its entirety. Supports are mentioned which include SiO2, the composition of which has been modified by impregnation with Al, Zr and/or Ti. Once the molding had been impregnated, it was dried and calcined. This known production method is highly elaborate, as modifying the base material by impregnation, drying and calcination makes substantial demands with regard to equipment and energy. Moreover, production times for modified moldings are distinctly longer.
It is known to produce pyrogenic mixed oxides by simultaneously reacting at least two different metals in the form of volatile metal compounds, for example chlorides, in a H2/O2 flame. One example of such an oxide is the SiO2/Al2O3 mixed oxide, which is produced by Degussa and sold under the name Aerosil(copyright) MOX 170. When producing Aerosil(copyright) MOX 170, a mixture of SiCl4 and AlCl3 is directly hydrolyzed in a flame. Corresponding silanes, such as, for example, methyltrichlorosilane, trichlorosilanes, etc., may also be used as a raw material instead of or in addition to the chlorides. (Degussa Technical Bulletin Pigments, No. 11: Basic Characteristics of Aerosil(copyright), pages 37 and 11-12; AT-A 195 893; DE-A 952 891; DE-A 25 33 925; DE-A 27 02 896. These documents each are entirely incorporated herein by reference.)
It is furthermore known to produce moldings based on pyrogenically produced silicon dioxide/aluminum oxide mixed oxide (EP-B 0 327 815). This document is incorporated by reference herein in its entirety. These mixed oxides contain at least 75 wt. % silicon dioxide and at most 25 wt. % aluminum oxide.
The present invention provides moldings based on pyrogenically produced mixed oxide, having the following physicochemical parameters:
The moldings according to the invention may be present as extruded moldings, extrudates or tablets. They may assume the form of cylinders, cylinders having rounded end faces, spheres, rings, wagon wheels, miniliths or other shapes conventional for fixed bed catalysts.
The present invention also provides a process for the production of moldings according to the invention based on pyrogenically produced mixed oxide having the following physicochemical parameters:
which process is characterized in that a pyrogenically produced mixed oxide is homogenized as desired with one or more compounds selected from the group of methylcellulose, methylhydroxyethylcellulose, wax, magnesium stearate, aluminum stearate or polyethylene glycol, with the addition of water; dried at a temperature of 70-150xc2x0 C.; optionally comminuted to yield a powder; the powder is compression molded to yield moldings; and then heat treated for a period of 0.5 to 10 hours at a temperature of 400 to 1200xc2x0 C.
The moldings according to the invention may be produced in stamping presses, eccentric presses, isostatic presses, extrusion presses, rotary presses or compactors.
Before pressing, in one particular embodiment of the invention, the mixture may exhibit the following composition:
Fracture strength, specific total surface area and pore volume may be adjusted to a certain extent by varying the quantities of starting materials and the compression pressure.
The present invention also provides a process for the production of moldings according to the invention based on pyrogenically produced mixed oxide having the following physicochemical parameters:
which process is characterized in that a pyrogenically produced mixed oxide is homogenized as desired with one or more compounds selected from the group of methylcellulose, methylhydroxyethylcellulose, wax, magnesium stearate, aluminum stearate or polyethylene glycol, with the addition of water; subjected to a kneading and shaping process; extruded; the extrudates are optionally chopped to the desired length by means of a chopping device; dried at a temperature of 70-150xc2x0 C.; and then heat treated for a period of 0.5 to 10 hours at a temperature of 400 to 1200xc2x0 C.
Any mixers or mills that permit good homogenization, such as, for example, blade mixers, fluidized bed mixers, centrifugal mixers or air-swept mixers, are suitable for performing the process according to the invention. Particularly suitable mixers are those with which the material being mixed may additionally be compacted, such as, for example plough bar mixers, pan mills or ball mills. Mixing and kneading, however, may also proceed directly in an extruder. The extrudates may also be produced in single or twin-screw extruders, extrusion presses, as well as in compactors. After homogenization, the product may be largely dried at 70-150xc2x0 C., such that, after an optional comminution operation, a pourable powder is obtained.